The big number of shared air victories (31) may reflect the total of air victories of the air unit in which Shestakov served in the Spanish Civil War during his service time. If he was a active pilot and took part in many air combats it is not all strange if his "shared kills" score is high. However it is difficult to tell how big his part was in those air combats in which he was credited with a "share kill". He might have actually contributed something to the "unit kill" or he may well have been just flying around without contributing anything significant to the kills credited "officially" as "shared kills" to him.

The system of shared air victories was very generous in Spanish Republican Air Force as well as in some other air forces - compare for example the French Air Force in WW2. Pilots of patrullas (swarms) or even whole escuadrillas (squadrons) could be credited by downing the one and same enemy aircraft. Thus many pilots could be credited by shooting down a enemy aircraft in a combat in which actually one enemy plane was shot down. I remember to have read about one French case in which as many as 8 pilots were credited with a "share kill" from a combat in which just one enemy plane was shot down. In that case there were pilots of two units involved so it produced 2 "unit kills" which even that was overclaiming 2:1. From the Soviet-Finnish Winter War I have read about one case in which as many as 12 Soviet pilots were credited with a "shared" air victory from the combat in which just one Finnish aircraft was shot down (claims of 25 IAP on 2.2.1940, IIRC). Later these shared claims were "upgraded" to the highly exaggerated claim that as many as 12 Finnish planes were shot down in that single combat!

With the shared claims it is difficult to tell if the pilot actually contributed to some significant way to the combat or was just flying around without getting a single shot on enemy aircraft claimed to be shot down. The generously given group/shared "kills" are known to lead to confused and often highly inflated air victory records.

I guess that any attempt to verify the decimal value of Lev Shestakov´s shared aereal kills in Spanish Civil War is hopeless task. There is even disagreement about his individual score. Some literary/web sources claim that his "full individual" claim score in Spanish Civil War were only 2 confirmed + 1 probable, others put it up to eight "confirmed" and some argue that it might have been as many as 11. So when even his "invidual score" is somewhat a puzzle it is even more difficult to find out how accurate the "shared score" actually was.

If you are really interested, you can try to browse Håkans Aviation Page about the airwar in Spain - see http://surfcity.kund.dalnet.se/scw.htm - there might be some details on Shestakov. There are also some good books about Soviet pilots and air advisors in Spanish Civil War - these surely have something on Shestakov and probably tell also something about his "kill score" in Spain.

Anyway I would not compare Shestakov´s score with those of real top aces of Spanish Civil War. For example J. García Morato´s 40 air victory record was based on indivdidual claims and is therefore more clear stuff than Shestakov´s 2½/8/11 individual (?) + 31 shared (?).

There is also the further issue of comparing the claims with recorded enemy losses. In that case J. García Morato´s score would be easier and more thankful individual case study than trying to find any "true" decimal value to Shestakov´s shared kills.

In total Lev Shestakov in Spain made more than 100 fighting departures, I participated in 36 air fights, personally I brought down 2 fighters (Me-109 and Fiat-32) and together with other pilot of 1 Me-109:
00.08.1937 1/2 Ме-109
22.12.1937 1 Fiat-32 Teruel
00.12.1937 1 Ме-109 Teruel